Does the driver bear responsibility for a traffic accident he caused after using the car horn to alert another driver who suddenly changed his lane, leading to the other driver colliding with concrete and other cars and being injured?

If the collision occurred due to the other party after you had warned them, and they took responsibility for their strong deviation which led to them colliding with concrete and then with other cars, then you are not responsible for anything that resulted from the accident. This is because you were not directly involved in causing the injuries and wounds; rather, you were an indirect cause, while the other driver was the direct cause. The jurisprudential rule states: "If a direct cause and an indirect cause combine, the ruling is attributed to the direct cause."